94 



The following simple modification of this classic experiment will be 

 instructive and will nsually set the student to thinking to some purpose. 

 I'repare several ri)und disks from (i]>a(]ue paper or light cardlioard. These 

 may he readily made of about the I'ight size with a guuwad cutter. Se- 

 lecting a broad leaved plant of suitable size, place a pair of the dislis one 

 on each side of the leaf opposite each other and in close contact with the 

 surface of the leaf, and fastened in place by ;i light si)ring c]i]i. smli as is 

 commonly used to fasten loose sheets of paper. Attach another pair of 

 disks in a similar manner but snpixirted from the surface of the leaf by 

 wax feet or other .supports so as tn allow a free circulation of air between 

 the dislvs and the leaf. Now select a cnuple of cover glasses, such as are 

 used in microscopic woi-k, of approximately the same size as the paper 

 disks and attach tliese in a similar manner so as to be in close contact 

 with the surface of the leaf. The iilant is nnw c.xiiosed to the sunlight and 

 after a time the leaf is teste<l tor starch in the usual way with iodine. 

 The result .is obtained recently by my own students is shown in tlie figure. 



As is seen the place of the oiiaijne disks is marked by a sjiot free from 

 starch .and ixiunded by ;i shai'ply defined line approximately .at the edge 

 of the disk ill both cases, showing of how little significance in this form of 

 the experiment is the question whether the stomata are covered or not. 

 On the oth( r hand, the glass disks are m.irked by only .-i small spot .at the 

 center without starch and the intensity of the starcli reaction sh.-ides oH 

 gradually frtMu the margin toward this central s])(.t so th.-it the boundary 

 of the disk is not evident. It is hardly necessary to add that some care 

 should be exercise*] in timing the exposure to the sunligbt in oi'der to get 

 the most striking results. The phenomena afford a neat little problem for 

 the student to explain. With a little thought most students can do this if 

 they understand cleaiiy the structure of tlie le;if. 



The exi)eriment may be fiirtiier elabor.ited with instructive results by 

 selecting le.aves with stomata deliintely distributed .-is upon only one side 

 of th" leal and liy using the disks. Iioth the glass and opacpie, singly 

 instead of in pairs. 



DcPauic Vmpersity, 

 Greencastle, Ind. 



